FIG. 217.—Telespectroscope with Camera for obtaining Photographs of the Solar Prominences. ] In order to obtain a photograph of the average solar spectrum, the camera replaces the observing telescope, and a heliostat is used, as in the ordinary way. The beam, however, should be sent through an opera-glass in order to condense it, and thereby to render the exposure as short as possible. Further, if an electric lamp be mounted as shown in Fig. 216, observations, similar to those originally made by Kirchhoff, of the coincidence on the various metallic lines with the Fraunhofer ones, can be permanently recorded on the photographic plate. The lens between the lamp and the heliostat is for the purpose of throwing an image of the sun between the carbon poles. The lens between the lamp and spectroscope then focuses both the poles and the image of the sun on to the slit. The spectrum of the sun is first obtained by uncovering a small part of the slit and allowing the image of the sun to fall on this uncovered portion, the lamp not being in action. When this has been done the light of the sun is shut off. The metal to be studied is placed in the lower pole; the adjacent portion of the slit is uncovered, that at first used being closed in the process. The current is then passed to render the metal incandescent. After the proper exposure the plate is developed and the spectra are seen side by side. Fig. 187 is a woodcut of a plate so obtained. If the spectrum of any special part of the sun, or the prominences, has to be photographed, then either a siderostat must be employed, or a camera is adjusted to the telespectroscope, as shown in Fig. 217. For the stars, of course, much smaller dispersion must be used, but the method is the same; and what has already been said by way of precaution about the observation of stellar spectra applies equally to the attempt to obtain spectrum photographs of these distant suns. INDEX. A. Aberration (_see_ Chromatic Aberration, Spherical Aberration) Absorption, general and selective, 403, 408; spectroscope arranged for showing, 409 Adjustment of the transit instrument, 238 ADJUSTMENTS OF THE EQUATORIAL (Chap. XXI.), 328 Achromaticity of Huyghen’s eyepiece, 110 Achromatic lenses, 84, 86 Achromatism, 126 Airy’s transit circle, 284 Alexandrian Museum, astronomical observations, 19 Alt-azimuth, 287, 289 Altitudes, instrument used by Ptolemy for measuring, 35 Aluminium, line spectrum of, 406; the sun, 417 Analyser for polarization of light, 443, 450 Anaximander, his theory of the form of the earth, 6; invention of the gnomon ascribed to him, 16, 17; meridian observations by, 25 Anchor escapement, 197 Angles of position, measurement of, 358-366, 372 Ångström, spectrum analysis, 402, 412; wave-lengths, 406 Annealing of lenses and specula, 121 Archimedes, clocks used by, 176 Arcturus, heat of, 385